\tikzstyle{vertex}=[draw,fill=black!15,circle,minimum size=20pt,inner sep=0pt]
\begin{tikzpicture}[very thick,level/.style={sibling distance=60mm/#1}]
\node [vertex] (r){$a$}
  child {
    node [vertex] (a) {$b$}
    child {
      node [vertex] {$c$}
    }
    child {
      node [vertex] {$d$}
    }
  }
  child {
    node [vertex] {$e$}
  };
\end{tikzpicture}